Sally Green recalls being put off writing at school: 'My English teacher wasn’t that good … generally it was driven out of me.' Photograph: Mark Allan

After JK Rowling's wizards and Stephenie Meyer's vampires, publishers and producers have now been spellbound by a first novel about witches written by a former accountant.

Sally Green, 52, had no desire to write until three years ago. Yet her supernatural thriller about witches living secretly among us in contemporary Britain has been snapped up by publishers in 36 countries, from Canada to Ukraine, who see her as a potential big hit, with stories that will appeal to teenagers and adults alike. Advances for a trilogy of novels are expected to earn the author about £1m.

The film rights have been bought by Fox 2000 and Karen Rosenfelt, who produced The Twilight Saga, inspired by Meyer's vampires.

In the story, black and white witches are divided by hatred but united by a fear of a boy called Nathan, who is descended from both sides – "wanted by no one; hunted by everyone".

Green, who lives in Warrington, Cheshire, with her husband and 11-year-old son, turned to writing only to fill the hours while her son was at school. She said: "I didn't imagine I'd finish the novel, never mind that it would get published. I'm blown away."

She recalls being put off writing at school: "My English teacher wasn't that good. I do remember having bits of inspiration, but generally it was driven out of me."

Half Bad, the first novel by Sally Green, is predicted to become the next Harry Potter or Twilight Saga

Having studied mining geology, she worked as an accountant until she became pregnant. Later she embarked on an Open University social sciences course. "That got my brain working again," she said. "I've often had ideas of stories but I really never believed I could write … I found I was staying up until 2am just writing." She later switched to a creative-writing course.

Claire Wilson, of literary agents Rogers, Coleridge & White, found Green's work among some 200 unsolicited manuscripts she receives a month. Within a few pages, she was gripped: "I was sitting bolt upright, realising it was something special … It was a strange combination of feelings – hugely excited and incredibly panicked that someone else might be reading it at the same time.

"I was completely electrified. I read it overnight and emailed Sally the next day." Green found herself signed up with an agency whose authors include Ian McEwan and Nick Hornby.

Wilson said: "It was an agent's dream come true … It grabs you immediately and doesn't let go from the very first page." She was particularly inspired by the novel's "shifting moral sands" where there is "no simple answer to who is a good guy and who is evil".

Penguin, which acquired the novel earlier this year, predicts it will do for witches what Twilight did for vampires. Ben Horslen, editorial director of its Puffin imprint, said: "What drew me to it immediately was 'the voice' … There's an almost George Orwell-like quality.

"It's 1984 with witches. It's really uncompromising. She creates this whole society that is fuelled by suspicion and where the hero's family are under surveillance by this government that claims to be benevolent but clearly isn't. It's Britain as it would be today, except that there is this hidden society of witches living among us, but unknown to us."